Round 1

Reviewer 1 Report

The paper provides an interesting simulation work about battery thermal management. The paper is prepared carefully. 

1) There are ten cells in your battery pack, is it possibe to give a general conclusion about the temperature distribution among these ten cells ? For example, the cells in the center have more thermal increasing during the use, and they have a slow cooling speed compared with other cells ? 

 

2) coule you please point out the main challenges of battery thermal management in power tool and the main contributions in this paper ? Compared with EV, power grid, what's the main challenge of battery thermal management in power tool ? (or what's the different point between the use of battery in power tool and other applications such as EV, power grid. ) Please also list cearly your own contributions in the paper. 

 

2) The simulation has many discussion about the battery charging phase. Although the paper is just a simulation work in the battery thermal management domain, the connection with other related works should be pinted out such as battery control, SOC estimation, battery prognosis. Especially, more recent works about the battery charging control are recommended to be added and discussed in the introduction, such as "Nonlinear extension of battery constrained predictive charging control with transmission of Jacobian matrix." International Journal of Electrical Power & Energy Systems 146 (2023): 108762.

Author Response

Response to Reviewer 1 Comments

The paper provides an interesting simulation work about battery thermal management. The paper is prepared carefully. 

Comment 1:

There are ten cells in your battery pack, is it possibe to give a general conclusion about the temperature distribution among these ten cells ? For example, the cells in the center have more thermal increasing during the use, and they have a slow cooling speed compared with other cells ? 

Reply:

Thank you for highlighting this aspect. Temperature difference of the cells in the center and the cells at the edges during the whole cycle is discussed in chapter 3.3 and shown in Figure 4. Cells in at the edges are 1 to 5°C (depending on cooling material and phase) cooler than the cells in the center. We have supplemented chapter 3.3 in order to present this aspect more clearly.

 

Comment 2:

Could you please point out the main challenges of battery thermal management in power tool and the main contributions in this paper ? Compared with EV, power grid, what's the main challenge of battery thermal management in power tool ? (or what's the different point between the use of battery in power tool and other applications such as EV, power grid. ) Please also list cearly your own contributions in the paper. 

Reply:

Thank you for the advice. Appropriate sections have been added in the introduction part (chapter 1) of the revised manuscript, see pages 2-3 in the manuscript respectively also added below.

“Due to the direct use at the end user, the small space inside the device as well as safety and usability requirements, cooling systems in battery-powered portable tools face different challenges compared to cooling systems in electric mobility or grid applications: For example, cooling systems have to…
(1) be powerful, as the comparatively small battery has to provide a lot of energy in a short time, especially when the tools are in use. This generates a lot of heat for a short time, which needs to be dissipated by the cooling system.

(2) help to achieve the shortest possible charging time. The duration of the charging process is determined on the one hand by the charging current (fast charging). In Powertools, however, it is precisely an excessively high temperature at the start of the charging process that restricts charging. Charging cannot be started immediately and thus the charging process is significantly prolonged. A charging process that is as short as possible and thus short downtimes of the devices are of great interest to the user.

(3) be lightweight and take up very little space. Portability and user-friendliness can only be achieved in this way.

(4) be maintenance free and have user independent control of both the cooling system and the battery management system and their interaction. It is essential that the control system is as simple and independent as possible, as the units are intended for end users.

(5) be safe. Equipment shall be designed to minimize the risk of personal injury and property damage from fire caused by overheated batteries.

(6) promote long battery life in the pack. This is important in order to maximize user acceptance, but also in terms of resource conservation and sustainability. This can be achieved through a cooling system which keeps the pack at the most comfortable temperature range for the cells.

Mastering these challenges is made possible by the interplay between the choice of the right cooling system and its correct design, tailored to the specific application, as well as a suitable interplay with the control of the battery by the battery management system, which also controls the charging and discharging currents with regard to the thermal development.

This study contributes to a better understanding of cooling systems in power tool battery packs. It helps to select a cooling material that fits exactly and thus a solution tailored to the performance requirements. This makes it possible to find an optimal compromise between high performance, safety, user-friendliness through short charging time and longevity with regard to cell aging. In particular, short charging times are addressed. By comparing different cooling materials with a phase change material, the behaviour of the materials in battery cooling systems can be better understood. The application-oriented simulation in the use charge cycle allows an insight into the effects of the cooling material used at different typical loads (discharging, loads, etc.). The study also provides an example of how simulation methods can be used to play through the complex interaction of cooling system, load sequence and safety and ageing requirements and to identify the adjusting screws for targeted optimization of the system.”

 

Comment 3:

The simulation has many discussion about the battery charging phase. Although the paper is just a simulation work in the battery thermal management domain, the connection with other related works should be pointed out such as battery control, SOC estimation, battery prognosis. Especially, more recent works about the battery charging control are recommended to be added and discussed in the introduction, such as "Nonlinear extension of battery constrained predictive charging control with transmission of Jacobian matrix." International Journal of Electrical Power & Energy Systems 146 (2023): 108762.

Reply:

Thank you for pointing this out. Appropriate sections have been added in the introduction (chapter 1) part of the revised manuscript, see page 2 in the manuscript. We have kept the section short because we think, as this work focuses on thermal management and helps to better assess this, but does not address how this can be managed through a management system.

Reviewer 2 Report

This report evaluates four different pack cooling systems. This report is detailed and reasonable and, with further modification, is suitable for publication in Applied Sciences.

1)    The introduction of the research background is not detailed enough. It is suggested to cite more research results to illustrate the significance and innovation of this study.

2)    There are large irregularities in the writing that need to be corrected.

3)    What is the reason for the difference in the performance of latent heat storage materials in the cool down I 1 and cool down Ⅱ stages?

4)  There should be a graph or table to compare the results of this report with those of the references.

Grammar and writing standards should be checked.

Author Response

Response to Reviewer 2 Comments

This report evaluates four different pack cooling systems. This report is detailed and reasonable and, with further modification, is suitable for publication in Applied Sciences.

Comment 1:

The introduction of the research background is not detailed enough. It is suggested to cite more research results to illustrate the significance and innovation of this study.

Reply:

Thank you for the note about the literature being too scarce. Unfortunately, there is very little suitable literature on the specific topic of cooling systems that are explicitly tailored to an application in portable power tools. However, we have added further parts to the manuscript in addition to the already existing literature section.

 

Comment 2:

There are large irregularities in the writing that need to be corrected.

Reply:

The manuscript was revised for spelling and language quality. We now hope to meet the requirements for a publication in Applied Sciences.

 

Comment 3:

What is the reason for the difference in the performance of latent heat storage materials in the cool down I 1 and cool down Ⅱ stages?

Reply:

Phase transformation temperature for the latent heat storage material used is at 39°C.
During Cool Down I phase (minute 8.5-81), the temperature of the cooling system is always above 39°C, so no phase transformation takes place during this phase.
In contrast, during Cool Down II, the system cools from 46°C to 26°C, the phase transition temperature is reached, and a transition from liquid to solid takes place. The energy released in this process significantly prolongs this phase (see also Figure 3 "Phase Change"). The matter is described accordingly in section 3.1 "Charging and cool down".

 

Comment 4:

There should be a graph or table to compare the results of this report with those of the references.

Reply:

Thank you for pointing this out. We agree, that a direct comparison of our results with those form references would be very interesting – and provide more insights. However, the study presented investigates the application of cooling materials in a cooling system in a specific use case. Therefore, the results cannot be readily compared with the results of other studies which do not target the same use case with a correspondingly similar load profile. Unfortunately, we are not aware of any correspondingly suitable studies with which a direct comparison would be possible. A comparison with results from other studies, which do not match, does not seem to us to be purposeful.

 

Comment 4:

Grammar and writing standards should be checked

Reply:

The manuscript was revised for spelling and language quality. We now hope to meet the requirements for a publication in Applied Sciences.

Reviewer 3 Report

The validation of numerical models is inadequate.

How can one obtain a solution that is free from grid constraints?

SOC is unsuitable for representing a variable.

It is advisable to consolidate the initial and boundary conditions.

How to compute u_trans?

Minor editing of English language required

Author Response

Response to Reviewer 3 Comments

Thank you for the kind revision and the points you mention. Unfortunately, the explanations of the revision are not very detailed. Nevertheless, we will try to answer the points mentioned as best as possible. If we misinterpreted some comments or missed your points, please let us know.

 

Comment 1:

The validation of numerical models is inadequate.

Reply:

Thank you for pointing this out. In the study, we use two models and validate them via a simplified measurement setup and use an optimization algorithm to further improve the fit of the measured battery parameters. An approach, which is common in battery simulation. Details on the validation are depicted in Appendix A in the manuscript. We are convinced, that the validation measurements describe the behavior sufficiently well to be able to make the statements and draw the conclusions we get from the results of the simulation study.
For simulation, we rely on the recognized commercial simulation software Comsol and do not use any special models or approaches - i.e., we use models proven in numerous other studies and applications.

Comment 2:

How can one obtain a solution that is free from grid constraints?

Reply:

The solution of our simulation study is certainly influenced by the fineness and type of the grid used. However, since we do not consider specific interface issues or very fine details, we believe that the quality of our grid is by far good enough to get reliable results – and allows us to draw corresponding conclusions.

 

Comment 3:

SOC is unsuitable for representing a variable.

Reply:

We are not sure whether we got this comment right.

The term “SOC” is used in the study, as it is also commonly used in battery science, for the state of charge of the battery. It reaches values between zero (battery empty) and one (battery fully charged). Accordingly, the term is also used in the battery simulation model used. We would like to point out that we are not using a novel model here. We use an existing model implemented in the Comsol software and which has confirmed its validity in numerous other studies and is very well documented. SOC is used as a term in the used Comsol battery model (see also Appendix 2 or Comsol documentation).

 

Comment 4:

It is advisable to consolidate the initial and boundary conditions.

Reply:

Thank you for pointing this out. Initial and boundary conditions are not specifically named as such in the study, but are provided in chapter 2 in the manuscript. The manuscript is not primarily intended for simulation experts, but for researchers and users with a battery and cooling system background. It has therefore been written in a way that is easy to understand for them and is not overloaded with complex simulation details and terminology. We added the initial and boundary conditions in a separate list in the appendix.

 

 

Comment 5:

How to compute u_trans?

Reply:

Thank you for this question. U_trans is used in the Comsol heat transfer model in solids used in the study. It accounts for the velocity vector of translational motion and is provided by other Comsol modules, where it is necessary. As no movement occurs, the term is of no importance in this study. We have nevertheless reported the term in the theory section to reflect the model as it is used in Comsol.

 

Comment 6:

Minor editing of English language required

Reply:

The manuscript was revised for spelling and language quality. We now hope to meet the requirements for a publication in Applied Sciences.

Reviewer 4 Report

This paper reported a simulation model of an 18 V power tool battery pack was developed to be able to evaluate four different pack cooling systems. The content is detail, it can be accepted. However, some minor issues should be improved.

 

1.      The introduction is too long, please simplify.

 

2.      Please abbreviate the journal name of the references.

 

3.      Some recent references may be included to enrich the manuscript, such as Journal of Energy Storage 2023, 68, 107744; Molecules, 2023, 28, 2108.

Language is fine.

Author Response

Response to Reviewer 4 Comments

 

 

Comments and Suggestions for Authors

This paper reported a simulation model of an 18 V power tool battery pack was developed to be able to evaluate four different pack cooling systems. The content is detail, it can be accepted. However, some minor issues should be improved.

Comment 1:
The introduction is too long, please simplify.

Reply:
Thank you for pointing this out. We are also of the opinion that an introduction should be as concise as possible. However, for the application-oriented topic considered in this study, it is essential to explain several things in order to understand the background - so we cannot make it too short.
Other reviewers unfortunately pointed out that important points were missing from the introduction and after consideration we had to extend the chapter again, contrary to our original intention and your advice.

Comment 2:
Please abbreviate the journal name of the references.
Reply:
Thank you for the hint – we will check with the journal and adjust the references accordingly.

 

Comment 3:
Some recent references may be included to enrich the manuscript, such as Journal of Energy Storage 2023, 68, 107744; Molecules, 2023, 28, 2108.
Reply:
Thank you for your suggestion and the and the proposed paper. We have significantly expanded the literature section in the revised manuscript.

Round 2

Reviewer 1 Report

The questions have been answered, the paper can be accpeted. 

Reviewer 3 Report

it can be accetped